docs: admin-guide: edid: Clarify where to run "make"

When both the documentation and the data files lived in
Documentation/EDID, this wasn't necessary, but both have
been moved to other directories in the meantime.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>